As the use of computing technology and computer networks (e.g., the Internet) continues to accelerate, the ease with which digital media and files can be copied has created a need for copy protection schemes. Typically, cryptographic systems permit only valid keyholders with access to encrypted data. However, once decrypted, it is difficult to track the reproduction or retransmission of such data. Thus, such schemes provide insufficient protection against unauthorized reproduction of information. A digital watermark can be inserted into a document to address this problem. Such a watermark may be visible and include an identification code permanently embedded in the data. That is, the identification code remains present within the data after any decryption process, and can be used to detect and protect against the unauthorized reproduction of the data. One example of a digital watermark includes a visible seal inserted into an image to identify the copyright owner. However, a watermark could also contain additional information, including the identity of the purchaser of a specific copy of the image, in order to better track the reproduction of the specific copy.
A number of schemes have been proposed for watermarking digital data. For instance, one watermarking procedure includes varying each copy of a document by a marginal amount, so that each copy of the document appears to be the same when viewed by the human eye, and such that each copy of the document is a unique variation. In such a system, although the slight variations are invisible to the human eye, the unique variations can be detected by analyzing the document and thus can be used to identify the origin a particular copy of the document. Whichever watermarking technique is used, it is generally preferable for the watermark to be difficult to remove without destroying the document. Doing so helps to ensure that copies of watermarked documents can be traced back to the specific version from which it was created.
Additionally, a need exists for copy prevention techniques that prevent photographs from being taken of displayed content, and this need has become particular pronounced due to the proliferation of digital cameras (e.g., mobile device configured with a digital camera). For example, many amusement parks offer a service in which visitors' photographs are taken during a particular ride or attraction (e.g., a roller coaster), and these photographs can then be sold to the visitors for a nominal fee. Oftentimes, previews of these photographs are shown to visitors to encourage them to purchase a copy of such photographs. However, due to the increased popularity of digital cameras (including mobile devices equipped with digital cameras), some users may simply take a digital picture of the preview copy using their digital camera, rather than paying for the copy of their personal photograph. As a result, some amusement parks have resorted to showing only low resolution previews of the photographs, while other vendors have stopped showing previews altogether. These solutions are not ideal, however, since high quality preview images can help encourage visitors to purchase a copy of their personal photograph.